C# 从小到大排列

            string str = "4231";
            int js = 1; var ss = "";
            for (int i=0;i<str.Length;i++)
            {
                var s0 = str[i].ToString();
                var s1 = (js).ToString();
                if (s0==s1)  //把1查找出来
                {
                    ss += str[i].ToString();
                    js++;i = -1;
                    if (js> str.Length)
                    {
                        break;
                    }
                }
            }